Instruction Manual

For example, the GPS receiver might determine that it is exactly

12,000 miles from satellite A, 12,700 miles from satellite B, and

13,100 miles from satellite C. At the same time, the aircraft’s encod-

ing altimeter might be indicating an altitude of 9,500 feet MSL. There

is only one point in space that satisfies these four measurements.

GPS DATA SIGNALS

Two of the primary types of signals that the GPS satellites broadcast

are almanac and ephemeris data. These signals enable the GPS

receiver to quickly lock on to the satellites in view. Otherwise, the

receiver would have to look for each of the 24 satellites to determine

which ones could be used.

Almanac data is very crude data which describes the approximate

orbital position of the satellites. Each of the 24 satellites transmits the

almanac data for all satellites, so a GPS receiver has only to listen to

one satellite in order to know which satellites are “visible” (in the sky)

at that particular time. Almanac data is good for about six months, so

when you turn the receiver off, then back on a month later, it will

know what satellites to look for.

Ephemeris data is very precise data which each satellite transmits to

tell the GPS receiver exactly where it is and what its orbital parame-

ters will be for about the next four hours. Each satellite transmits its

own unique ephemeris data.

Appendix F

F-2

GPS Primer

Appendix F

12,000 NM

12,700 NM

13,100 NM

9500 FT

MSL

NOT TO SCALE